GRM-144: feat(runner): enable IPv6 in rootless Docker via pasta network driver

AGENTS.md

@@ -57,7 +57,7 @@ CI also runs a best-effort `make workflow-dryrun` step (skipped if act_runner is
 ## Architecture
 
 - **Python CLI** (`src/grm/`) — Click-based CLI that delegates to Ansible
-- **Ansible Role** (`ansible/roles/gitea-runner/`) — Idempotent role for rootless Docker runner setup
+- **Ansible Role** (`ansible/roles/gitea-runner/`) — Idempotent role for rootless Docker runner setup with pasta networking (IPv6 support)
 - **devx package** (installed from git) — Reusable CI/CD tools: auto-merge, post-merge, release, publishing, molecule distribution, PR reviews, failure notifications
 - **Versioning** (`cliff.toml`) — git-cliff configuration for automated semver versioning from conventional commits
 

ansible/roles/gitea-runner/defaults/main.yml

@@ -53,3 +53,10 @@ docker_apt_source_line: >-
   {{ ansible_facts['distribution_release'] }} stable
 # Set to false in CI/molecule to skip rootless daemon startup (needs kernel userns)
 docker_rootless_setup: true
+
+# Rootless Docker network driver: "pasta" (IPv6 support) or "slirp4netns" (IPv4 only)
+# pasta has proper outgoing IPv6 support; slirp4netns does not (known limitation).
+docker_rootless_net_driver: "pasta"
+
+# IPv6 subnet for rootless Docker containers (ULA range, not routable on internet)
+docker_ipv6_cidr: "fd00:dead:beef::/48"

ansible/roles/gitea-runner/tasks/rootless_docker.yml

@@ -35,6 +35,7 @@
     name:
       - uidmap
       - slirp4netns
+      - passt
       - fuse-overlayfs
       - docker-ce
       - docker-ce-cli
@@ -57,6 +58,7 @@
       - docker
       - docker-compose
       - slirp4netns
+      - passt
       - fuse-overlayfs
       - rsync
     state: present
@@ -106,12 +108,48 @@
     group: "{{ gitea_runner_service_user }}"
   when: docker_rootless_setup
 
-- name: Configure rootless Docker to disable IPv6
+- name: Ensure systemd user override directory exists
+  ansible.builtin.file:
+    path: "{{ gitea_runner_home }}/.config/systemd/user/docker.service.d"
+    state: directory
+    mode: "0755"
+    owner: "{{ gitea_runner_service_user }}"
+    group: "{{ gitea_runner_service_user }}"
+  when: docker_rootless_setup
+
+- name: Configure rootless Docker to use pasta with IPv6
+  ansible.builtin.copy:
+    dest: "{{ gitea_runner_home }}/.config/systemd/user/docker.service.d/override.conf"
+    content: |
+      [Service]
+      Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_NET={{ docker_rootless_net_driver }}"
+      Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=implicit"
+      Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_FLAGS=--ipv6"
+    mode: "0644"
+    owner: "{{ gitea_runner_service_user }}"
+    group: "{{ gitea_runner_service_user }}"
+  register: docker_network_override
+  when: docker_rootless_setup
+
+- name: Reload systemd user daemon if network config changed
+  ansible.builtin.command: systemctl --user daemon-reload
+  become: true
+  become_user: "{{ gitea_runner_service_user }}"
+  environment:
+    XDG_RUNTIME_DIR: "/run/user/{{ gitea_runner_uid }}"
+  changed_when: true
+  when:
+    - docker_rootless_setup
+    - docker_network_override is changed
+
+- name: Configure rootless Docker daemon with IPv6 enabled
   ansible.builtin.copy:
     dest: "{{ gitea_runner_home }}/.config/docker/daemon.json"
     content: |
       {
-        "ipv6": false
+        "ipv6": true,
+        "ip6tables": true,
+        "fixed-cidr-v6": "{{ docker_ipv6_cidr }}"
       }
     mode: "0644"
     owner: "{{ gitea_runner_service_user }}"
@@ -128,7 +166,7 @@
   changed_when: true
   when:
     - docker_rootless_setup
-    - docker_ipv6_config is changed
+    - docker_ipv6_config is changed or docker_network_override is changed
 
 - name: Wait for rootless Docker daemon to be ready
   ansible.builtin.command: docker version

docs/tech/architecture.md

@@ -173,9 +173,21 @@ Each runner operates under a dedicated unprivileged system user. The Docker daem
 
 - User namespace mapping via `/etc/subuid` and `/etc/subgid` (range: 100000-165535)
 - Rootless Docker socket at `/run/user/<UID>/docker.sock`
-- `slirp4netns` for user-mode networking
+- `pasta` for user-mode networking with IPv6 support (replaces `slirp4netns`, which lacks outgoing IPv6)
 - `fuse-overlayfs` for rootless container storage
 
+The rootless Docker daemon is configured via a systemd user override
+(`docker.service.d/override.conf`) that sets:
+
+- `DOCKERD_ROOTLESS_ROOTLESSKIT_NET=pasta` — use pasta as the network driver
+- `DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=implicit` — pasta's native port forwarding
+- `DOCKERD_ROOTLESS_ROOTLESSKIT_FLAGS=--ipv6` — enable IPv6 routing
+
+The daemon.json enables IPv6 with a ULA subnet (`fd00:dead:beef::/48`)
+for container addressing. This ensures runner containers can reach
+both IPv4 and IPv6 services (e.g., the Gitea registry) without
+per-workaround DNS hacks.
+
 Containers launched by the runner never have root access to the host. The rootless Docker daemon is started as a systemd user service and persists via lingering.
 
 ### Secret handling